If you’re looking to get fit in a hurry, you should hit high intensities often. But understand that you are getting fit by only attacking one side of the equation, and there’s a limit to how fit you can ultimately get this way (not to mention a greater risk of injury and overtraining). Complete aerobic fitness happens when you put in the work at low intensities over time and develop the “low end” aerobic adaptations to support the higher intensity work.

Jason Dierking, assistant director for sports performance at the University of Louisville, explains how training load numbers help strength coaches and sport coaches keep student-athletes fit throughout the long season without overtraining them.